Project: ECOfamily Civic and its Aero Hitch Box / Boat Tail

[freebeard]

The conversation I had with aerohead about smoke testing on public roads starts here.

The biggest problem I see is the mismatch in tumblehome between the Kammback and the hitch box. The Kammback is cardboard. The hitch box is aluminum.

There was more to that thought. Hmmm....

[Xist]

I certainly do not want anyone pulled over by angry law enforcement--or facing attack helicopters!

Yet...

Could you retrofit your wiper? It already goes from side to side. Maybe if you pulled the blade and put a wheel on it...

[freebeard]

I was thinking an articulated wiper arm off a city bus or something (at the stagnation point). I never worked out how to have the smoke nozzle travel along the arm, but a simple start-stop control would be enough to get one one's half-body of revolution.

Heh, just have three or four nozzles spaced along the arm.

[christofoo]

On the 14th I did an A-B coast-down test of a full Kammback and found it was worse than none. (A- Aero Hitch Box, B- Aero Hitch Box plus full Kammback.) I'm hesitant to report in detail since there were a bunch of problems with the test (10 MPH winds with 17 MPH gusts, and cardboard and duct tape Kammback was falling apart, I only coasted down from 45 MPH instead of 70). Although the data was crummy and I was still learning how to do a quality coast-down, I also did a tuft test and felt that the results combined with tufts flowing into the gap indicated that gap filler needed to be addressed first. No point in getting into detail on the Kammback until the gap is filled.

So this Thur the 20th I did a much more detail-oriented A-B-A test with a gap filer. A- Aero Hitch box; B- Aero Hitch Box plus gap filler on the sides and top. I will have to start a new thread sometime about coast-down testing to discuss the many details, and I'm not finished analyzing my data either (and I may need to pick an even better road before I claim definitive accuracy), but I think I've applied due skepticism to the following chart and I think it holds water, at least in relative A-B-A terms, (not absolute accuracy):


A- average: .360
B- average: .335
Improvement: 7%

(Reminder: I think the 7% improvement I'm reporting is supportable, but I know I have a few more items to address to get a good absolute Cd, so take .335 with a grain of salt for now.)

So the gap filler appears to be mission-critical, and I've started fab on a road-worthy version. I would suspect the gap filler becomes even more critical with increasing yaw angle, so the 7% might even be lower than real-world.

Although I thought the spandex idea in the Aero Trailer TDI thread would probably work well for me, I wanted to try coroplast first. With a little luck I'll have some pictures to post early this week.

[California98Civic]

I'm glad you're still at it. I'm working on my own coast down testing and so I'd be happy to see your thoughts. I too considered testing even when there were significant winds, but I changed my mind.

Re: your chart. The yellow is with gap fillers and box and the blue is the box without the fillers? How did you calculate Cd? Because the stock Cd listing for the 1996-2000 Civic coupe was 0.32, while the hatch was something like 0.36 ... so something just does not look right, or else I am not getting it. You recorded speeds or distances and calculated Cd somehow? Did you start by assuming 0.36?

[Xist]

Originally Posted by christofoo

Well, it's been on my mind... I'm not sure how practical or workable this is though.

My pet idea is salvage an ultra-sonicator from a humidifier, apply it to fluorescent chalk instead of water, get a powerful blower and duct it into a 1" diameter vacuum cleaner hose, or something like that. Strap it to the roof, run the strap around through window cracks so it can be manipulated from the cab. But the hose would alter the air flow. Routing along the base of the front windshield would be the best compromise for on-road test. Hmm. Maybe I could use a 12V air compressor and make it with smaller air hose instead of the blower and vacuum cleaner hose.

Better yet, if I could wait for a day with a 30 MPH steady wind and minimal gusts I could find a place to park the car, preferably a wide-open lot, and use nature's wind-tunnel, and put the 'smoke' generator on a wand. Also need a weather vane, and preferably an anemometer.

I was thinking about this today, it just took a little while to find this discussion, but you already had better ideas that I had. I just wanted to share this thread that I found in the process: http://ecomodder.com/forum/showthrea...tor-10488.html

[christofoo]

Originally Posted by California98Civic

I'm glad you're still at it. I'm working on my own coast down testing and so I'd be happy to see your thoughts. I too considered testing even when there were significant winds, but I changed my mind.

Re: your chart. The yellow is with gap fillers and box and the blue is the box without the fillers? How did you calculate Cd? Because the stock Cd listing for the 1996-2000 Civic coupe was 0.32, while the hatch was something like 0.36 ... so something just does not look right, or else I am not getting it. You recorded speeds or distances and calculated Cd somehow? Did you start by assuming 0.36?

Blue is with the gap fillers (reduction in Cd), and yellow is without gap fillers. The calc is very similar to the Instructable, basically model and minimize Chi-squared. Crr shook out to be 0.012 BTW. Using a GPS for data capture.

Yes, 0.36 would be a surprisingly high figure for this car, even if the AHB was hurting it. But don't be alarmed just yet, I know I have at least one gremlin in my data since I coast better traveling east than west. There are several exercises I need to finish: error propagation on bumps and wind, GPS hysteresis, hill cross check, Euler method step size ... I'd like to start a new coast-down thread sooner than later, I'm just swamped for the next two weeks. But it's my kind of physics problem, it should be fun.

For the moment the point is that in A-B-A the gap filler had a significant benefit.

[freebeard]

...error propagation on bumps and wind, GPS hysteresis, hill cross check, Euler method step size...

I thought it would be as easy as going to H. B. Van Duzer Forest State Scenic Corridor* about 4 am, with a jury-rigged light plane pitot tube.

*long straight downhill 2-lane with 80' fir trees growing right up to both sides of the road.

[aerohead]

Originally Posted by christofoo

I actually think if the Aero Hitch Box fails to improve drag that I come out behind, since I already had that configuration with cheaper commercial components (rack and bag). But let's not get pessimistic.

My thoughts at this point:

1. The excess "stock" turbulence certainly is a mystery.
2. Tuft testing works well if the shape you're tufting is supposed to be an airfoil, i.e. where the body surface coincides with the relevant flow fields. I think tufts are intrinsically hard to interpret here because the shape I've got has gaps and so there are important flow fields that we can't see with tufts. A smoke wand would be much better.
3. I was dismayed to find the contrast of the tufts on top of the Kamm made them invisible in all of my youtube posts. Here's a still from the raw data. The only time you can see the tufts really well is when the car passes under a tree.
   What I wanted to know is whether the flow over the Kamm is smooth; I'm still not sure. It looks like parts of it are smooth, but I think that one weird tuft is real (not stuck on something).
4. The turbulence under the Kamm seems to be fed from the side of the car. I think once again we have higher pressure on the side. I'm still scratching my head about this, but it could simply be that the shape is 'astigmatic,' meaning the roofline curves before the side, as opposed to being rotationally symmetric. I also wonder if it has something to do with my grille block.
5. One theory is that the Kamm needs to have a lower angle. I might even have to raise the Aero Box lid to make the car's target profile more rotationally symmetric. My competing theory is that it just needs to be a full Kammback, from the roofline to the bumper. And of course for that to work properly it needs rounded edges.
6. No evidence of baffling having created an improvement. No evidence of a vertical air flow being a factor in any configuration.
7. No evidence of the side detachment garnish creating an improvement. Maybe I need to start the form further forward on the car and make the trailing edge flat like the box side instead of rounded like the car side.
8. The rear wheel skirt does help against the turbulence found at the side of the rear bumper, but doesn't affect the side of the box. I've always wondered whether the slick wheel cover and skirt are redundant, now I have my answer.

So there's more work to do.

The flow on your Kamm extension has flow which looks just like what Kamm had on his Kamm.
The uppermost tufts on the side are pointing into the low pressure on top of the roof,just like all of Kamm's K-form cars did.
Everything else looks okay excepting the vertical body side discontinuity between side extension and cargo box.But you were aware of this when you did it so no biggy!
The re-attachment onto the box top looks real good,and the sides will clean up when you have time to do a more proper side extension.
Everything else is do to the gap and void.
Lookin' good!

[aerohead]

Originally Posted by christofoo

Extended the Kammback (rear windshield cover) to the rear bumper, also I added a baffle to stop flow from the side, (also I found a less bumpy road).



So this configuration is:

• Full Kammback
• Rear wheel skirt
• Baffles between the car and the box, both horizontal and vertical.
• Aero Hitch Box
• EDIT: also I put the passenger side mirror back on, since I'm blocking 90% of the rear-view with the cardboard

Results:

I think that looks a lot better. The full Kammback always seemed like the solution that just had to work.

Video, cruising speed ranges a little from 30-40 MPH:

Well, I mean it still isn't perfect. I still need to find a form that fixes the turbulence on the side of the Aero Hitch Box, if possible.

But I think my next task is Kammback fab. (And duct tape gum removal... any pointers?)

EDIT: I forgot to mention; while I was scouting the road for this test this morning, I saw the sweetest full aero shell on a recumbent bike (velomobile). I wish I'd had time to snap a picture, but it was headed the other way. I think it was going at least 30 MPH on a very slight uphill. I wished I could have stopped the guy and asked if he made it himself, and if it was for a competition or not. I was so jealous.

The flow looks great! You're 'THERE'!
You could actually use the cardboard and tape, along with some protection for the car and actually fiberglass over the cardboard to create a composite Kammback.I've done this.
After the glassing,a good soaking in a childs wading pool will turn the cardboard to mush,washing away to leave the shell.
If need be,you can add a bit of stiffening to the inside.
A Plex or Lexan rear windshield would also improve the airflow.And it would also provide stiffening.
It's good that you restored the passenger side mirror,as you can now appreciate the new blind-spot.
This project has an enormous utility coefficient.